FI98537C - Method for separating sodium hydroxide in white liquor - Google Patents

Abstract

The present invention relates to a method of processing white liquor (3) obtained from the causticization step of a pulp mill, in which method the infeed white liquor (3) principally contains sodium hydroxide (6) and sodium sulfide (7). The method according to the invention is characterized in that the sodium hydroxide (6) contained in the white liquor (3) is separated from the white liquor (3) either entirely or partly by means of a diffusion dialysis process (4). The invention is further characterized in that the sodium hydroxide (6) separated from the white liquor (3) is advantageously entirely returned back to the chemical circulation of the pulp mill. <IMAGE>

Description

98537

METHOD FOR THE SEPARATION OF SODIUM HYDROXIDE IN THE WHITE PIPE - FÖRFARANDE FÖR SEPARERING AV SODIUM HYDROXIDE UR VITLUT

The present invention relates to a process for the separation of sodium hydroxide in white liquor according to the preamble of claim 1.

In the future, the production of TCF (to-10 tally chlorine free) pulps without chlorine chemicals will increase significantly. With the transition to oxygen-peroxide-based bleaching methods for bleaching plants with pure sodium hydroxide, the need for NaOH will increase. On the other hand, the production of TCF pulps allows the water circuits of the bleaching to be closed and returned to the chemical cycle. When such cycles are closed, sodium enters the chemical cycle in excess, but in a form that is difficult to recover.

Bleaching of TCF pulps requires pure NaOH to keep the consumption of other bleaching chemicals to a minimum. Therefore, mills need to buy the pure lye needed by the bleacher. When the pulp mill bleaching plant and the chemical cycle are closed, an excess of sodium is formed in the chemical cycle, whereby the production of NaOH directly from the chemical cycle becomes topical.

* · · ** J Pure lye, NaOH, is used at the plant mainly in the bleaching of pulp II "., In its actual alkaline stage, and, if necessary, in other temporal stages of bleaching, in the production of sodium hydrofluorite, in chemical make-up chemistry. as a cabbage and neutralizing agent Pure lye is also used • i · • V for flue gas scrubbing Sodium sulphite from scrubbing • · ·: Na2SO3 can be returned to the chemical cycle.

35 The main method of preparing sodium hydroxide according to the prior art is the electrolysis of sodium chloride to chlorine and the so-called equivalent to sodium hydroxide. NaOH can also be prepared in other ways, e.g. by cooling-crystallization-causticization, in which case, by cooling from the green liquor, the sodium carbonate therein is crystallized, which is causticized. The process henceforth requires 5 parallel causticization lines up to the separation of the liquor.

It is also known to prepare NaOH by electrodialysis decomposition of sodium sulphate into NaOH and sulfuric acid, in which case the lye formed as a product is about 15%.

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The basic unit for the preparation of NaOH, which is known in the art, is the electrolytic cell. In it, the very pure NaCl solution is decomposed with direct current. There are two different types of cells; mercury cell and membrane cell. There are 15 cells in the plant, usually 50 to 100 in series. The mercury cell consists of two parts, namely the primary cell and the secondary cell. The primary cell has a titanium anode (positive charge) that generates chlorine gas and a mobile mercury cathode that amalgams the resulting sodium with mercury. The amalgam flows into a 20 secondary cell where water is added to it, whereupon the amalgam decomposes into sodium hydroxide, hydrogen gas, and back to mercury. The mercury is returned to the cell. Sodium hydroxide is recovered as a 50% aqueous solution. In the membrane cell, the anode and cathode states are separated by an ion-selective membrane, a membrane. The membrane .U 'only allows sodium ions to pass through. This produces chlorine at the anode and hydrogen and sodium hydroxide at the cathode. Sodium hydroxide is recovered at 20% by weight, but must be evaporated for storage and transport.

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It is an object of the present invention to provide a solution with which «·« *. from the chemical cycle, pure NaOH can be produced from the required amount of white liquor, eg for the needs of the bleaching plant, in which case no purchase liquor is needed and the chemical cycle is equilibrated. Characteristic features of the method according to the invention are set out in the claims.

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According to the invention, the required amount of pure sodium hydroxide can be separated from the white liquor by diffusion dialysis treatment without changing the sodium / sulfur balance of the circuit. The product yields about 8% lye, which can be used as such for bleaching. The second fraction is the sodium sulphide fraction (pH over 10), which can be fed to the soup to give the so-called sulfur-containing soup to improve pulp properties and yield.

According to the invention, the diffusion dialysis process can take the required fraction of white liquor from the pulp mill's own chemical cycle and feed it to the diffusion dialysis equipment, resulting in pure about 8% lye and sodium sulfide fraction which can be fed to the soup. The real advantage of diffusion dialysis age-15 treatment compared to the lye production methods described above is its low energy consumption. In diffusion dialysis, energy is only consumed for pumping the feed solutions. Electrodialysis consumes about 3000 kWh / t of 100% NaOH electricity, which is slightly less than the traditional electrolysis method for lye production. On the other hand, cooling crystallization-causticization correspondingly requires a separate causticization line, whereby the equipment investment is high.

Diffusion dialysis equipment is easy to place in the factory 2J5. '. chemical cycle because its space requirements are reasonable.

* «» ·· It is easy to increase or decrease the equipment according to the production «« · · j, \\. The process does not require separate control, but it, ···, basically works on its own. In addition, in the future t «· when closing the cycles in the chemical cycle, there will be an excess of sodium 3Q,,. The diffusion dialysis device according to the invention

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compared to the prior art, the treatment is the only way to utilize • »· * · * * sodium cycles.

The cation exchange membrane is a polymer matrix structure having 35. bound to cationic groups. The polymer matrix is usually polystyrene, polyethylene, polysulfone, polytetra-4 98537 fluoroethylene or fluorinated ethylene polymers. The support structure can be, for example, styrene. The cationic group is a sulfite or carboxylic acid. The cation exchange membrane, as already mentioned above, is selective for cationic groups, in this case sodium ion. However, the hydroxyl ion passes through the membrane, while other anions do not. By varying the film density, ion exchange capacity, the amount of support structure, the transport properties of the film can be varied and thus the desired base and salt concentrations in the product can be optimized. The white liquor obtained from causticization after clarifier contains sodium hydroxide as well as sodium sulfide. Sodium ion diffuses through the cationic membrane, while sulfide ion and other anions do not.

The invention will now be described in more detail, by way of comparison results based on laboratory test runs, with reference to the accompanying drawing, in which

Figure 1 shows a process diagram of a method application according to the invention,

Figure 2 shows the principle of a diffusion dialysis apparatus suitable for the method according to Figure 1.

2J5. . Figures 1 and 2 show a method application and an apparatus consisting of a film pack containing cationic films. (4) and water (5) and white liquor (3) supply pumps and • · supply and product tanks. The film pack consists of only the required number of cationic films (4), which are selective. cations. On the other hand, the membrane (4) is also very selective for • · <: .f H ions and their diffusion through the membrane is very strong. The corresponding salts, on the other hand, diffuse slowly through the membrane and the cationic membrane (4) is passive with respect to them. From the point of view of the operation of the apparatus, it is advantageous to regenerate 35; as much lye as possible from the white liquor. On the other hand, the size of the equipment then increases, so that the desired product concentration

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5 98537 tio and the hardware size must be optimized according to the needs of each plant. However, the purpose of the process is to regenerate at least more than 60% of the lye so that the amount of sodium sulfide in the product stream is as small as possible.

5

A typical composition of white liquor is as follows:

NaOH 80-100 g / l

Na2S 50 ... 65 g / l

Na 2 CO 3 20 ... 30 g / l 10 Na 2 SO 4 5 ... 6 g / l

Na2S2O3 0.1 g / l other 0.2 g / l

According to the invention, after causticization (2), the required amount of white liquor is taken and fed to a diffusion dialysis cell. Water is fed into the cell upstream. The resulting pure lye fraction (6) is preferably recycled back to the bleaching (8). Correspondingly, the sulphide fraction (7) is preferably recycled back to the digester (9) and further to the recovery boiler (1). Thus, both fractions return to the chemical cycle through evaporation (10). The process preferably operates on the countercurrent principle, with water (5) fed into the pack from above and white liquor (3) (according to a reference result based on laboratory test runs) at concentrations corresponding to vx = 100 g / l. NaOH and Na 2 S s ·, = 60 g / l from below. The sodium (Na +) in the white liquor (3) diffuses through the cation membrane (4) into the water stream. (5), whereby the lye fraction (6) comes out of the process al-'* · * according to the comparison result at concentrations v2 = 75 «« g / l NaOH and s2 = 15 g / l Na2S. On the other hand, the sodium sulphide (7) in the white liquor (3) remains in the feed stream and comes out

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• · 'process from the top of the pack at corresponding concentrations v3 *. · * = 25 g / l NaOH and s3 = 45 g / l Na2S. The feed ratio of water (5) to white liquor: (3) is preferably 1.5: 1, the feed rate of white liquor is 1.6 1 / h / m 2 and the process temperature is about 20 ° C.

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The results of the test runs for white liquor are shown in Tables 1-3. Based on the values in the table, Table 4 of the driving parameter graph has been prepared, from which the available product concentrations and quantities as a function of the feed rate can be read. The results show that the process works reliably as required by the invention. In Annex 1, the material balance obtained by the method according to the invention at a mill producing 500,000 t / a of pulp at a consumption level of 30 kg NaOH / ts has been calculated on the basis of the test run results. The amount and concentrations of the two fractions obtained by the diffusion dialysis treatment according to the invention, pure NaOH and sulphide fraction, can be changed during the run by adjusting the ratio of feed and water flows, see Table 4.

According to the invention, it is further possible to use the so-called oxidized white liquor. When oxidized white liquor is used, pure lye is obtained as a second fraction and sodium thiosulfate fraction is obtained as a second.

The invention further relates to the use of diffusion dialysis to separate sodium hydroxide in white liquor from white liquor.

| ! It will be apparent to one skilled in the art that the various 25; Embodiments are not limited to the above examples, but may vary within the scope of the appended claims.

• «« • · * • »t« • «« • · · • • • · · · · · ·

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Claims (8)

A process for treating white liquor (3) formed during the causticization of the pulp mill, in which process white liquor (3) preferably consists of sodium hydroxide (6) and sodium sulfide (7), characterized in that the sodium hydroxide (6) the white liquor (3) is separated from the white liquor (3) either in whole or in part by means of diffusion dialysis treatment (4). 10 2. A process according to claim 1, characterized in that the sodium hydroxide (6) which is separated from the white liquor (3) is presumably wholly returned to the pulp mill's chemical circulation. 15 3. A process according to claim 1 or 2, characterized in that the sodium hydroxide (6) which is separated from the white liquor (3) is at least partially returned to the pulp mill's processes, presumably for the bleaching, flue gas washing and / or the production of sodium hypochlorite. 4. A process according to any of the preceding claims, characterized in that the sodium sulfide (7) which is separated: · from the white liquor (3) is at least partially returned to the boiling process. 5. A process according to any of the preceding claims characterized by: T: characterized in that the sodium hydroxide (6) is preferably separated by diffusion dialysis treatment (4). Sodium sulfide (7). · · · · · · · · · · · · · 6. A process according to any preceding claim, characterized in that the amount of sodium hydroxide (6) which is separated from the white liquor (3) by means of diffusion dialysis (4) is at least 50% of the total amount of sodium hydroxide (6). in white liquor (3). 98537 Process according to any of the preceding claims, characterized in that the concentration of the sodium hydroxide (6) which is separated from the white liquor (3) by means of diffusion dialysis treatment (4) is about 0.5 - 3.0 mol / l. 5 8. Use of diffusion dialysis (4) to separate sodium hydroxide (6) from white liquor (3). »• · · • ··· • * · * ·» • · • · «* t • · · · · • · · 0: II